Not found what you are looking for? Please ask a question or give feedback by emailing (firstname.lastname@example.org) or phoning (0131 650 3302) the EDINA Helpdesk. ] to transform between OSGB36 and WGS84. These range from very accurate (+/- 2m) to relatively inaccurate (+/- 35m), depending on where you are. NOTE: If you do not select a transformation, ArcGIS will still transform the data but not do a datum shift with a resulting accuracy of over 100m [ Usually the recommended transformation to use that is built into ArcGIS is the OSGB_1936_To_WGS_1984_Petroleum one (this uses the same parameters as the 7 parameter shift recommended by Ordnance Survey). This has an accuracy of +/- 5m. By default ArcGIS does not contain the OSTN02 transformation from Ordnance Survey that should be used to convert data from OSGB36 to WGS84 for the highest accuracy, however in ArcGIS 10.x it is simple to implement (note you only need to do this once): d) On reloading ArcGIS (or just reopening the Trransformations dialog) you will notice a transformation available if you choose OSGB36 to WGS84 called Source.